Note 8 - Common Stock (Details Textual)					3 Months Ended
	Feb. 12, 2018 USD ($) $ / shares shares	Nov. 08, 2017 USD ($)	May 22, 2017 USD ($) $ / shares shares	Jun. 20, 2016 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) shares
Proceeds from Issuance of Common Stock					$ 33,236,000	$ 31,503,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares					642,622
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares
Debt Instrument, Unamortized Discount, Total					$ 1,466,000
Class of Warrants or Rights, Exercises in Period | shares					0	0
Allocated Share-based Compensation Expense, Total					$ 753,000	$ 372,000
The 2016 Loan Agreement [Member]
Debt Issuance Costs, Net, Total				$ 90,000
Amortization of Debt Discount (Premium)						35,000
The 2017 Loan Agreement [Member]
Debt Issuance Costs, Net, Total					786,000
Amortization of Debt Discount (Premium)					77,000
Debt Instrument, Unamortized Discount, Total					$ 1,466,000
June 2016 Issuance Related to 2016 Loan Agreement [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares				100,402	100,402
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 4.98	$ 4.98
Warrants and Rights Outstanding				$ 350,000
Warrant Term				10 years
May 2017 Issuance Related to 2017 Loan Agreement [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares			222,049		222,049
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares			$ 9.50		$ 9.50
Warrants and Rights Outstanding			$ 940,000
Warrant Term			10 years
Allocated Share-based Compensation Expense, Total					$ 0	$ 0
May 2017 Issuance Related to 2017 Loan Agreement [Member] | Measurement Input, Expected Dividend Rate [Member]
Warrants and Rights Outstanding, Measurement Input			0
May 2017 Issuance Related to 2017 Loan Agreement [Member] | Measurement Input, Price Volatility [Member]
Warrants and Rights Outstanding, Measurement Input			55.1
May 2017 Issuance Related to 2017 Loan Agreement [Member] | Measurement Input, Risk Free Interest Rate [Member]
Warrants and Rights Outstanding, Measurement Input			2.25
May 2017 Issuance Related to 2017 Loan Agreement [Member] | Measurement Input, Expected Term [Member]
Warrants and Rights Outstanding, Measurement Input			10
Vendors and Contractors Warrants, October and November 2014 Issuance [Member]
Class of Warrants or Rights, Exercises in Period | shares					0
Class of Warrant or Right Number of Securities Called by Warrants or Rights Cancelled In Period | shares						0
February 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues | shares	11,500,000
Share Price | $ / shares	$ 3
Proceeds from Issuance of Common Stock	$ 34,500,000
Proceeds From Issuance of Common Stock, Net	$ 32,225,000
November 2017 ATM Facility [Member]
Stock Issued During Period, Shares, New Issues | shares					267,526
Proceeds From Issuance of Common Stock, Net					$ 1,136,000
Aggregate Offering Price		$ 25,000,000
ATM Facility [Member]
Stock Issued During Period, Shares, New Issues | shares					208,277
Proceeds From Issuance of Common Stock, Net					$ 1,011,000